1. Field of the Invention
The present invention relates to an electromigration evaluation circuit for use in evaluating electromigration resistance of a to-be-evaluated device.
2. Description of the Related Art
Electromigration (hereinafter referred to as EM) is a phenomenon wherein metal ions migrate as a result of an exchange of momentum between electrons and metal ions flowing to through a conductor. This EM phenomenon may occur, in a case where a current of a comparatively large current density flows to metal wiring of a semiconductor integrated circuit, etc., for a long time. If the EM phenomenon occurs, a problem is that the electric resistance of the metal wiring varies, and thus causing a breakage in the metal wiring.
To prevent such a problem, it is necessary to accurately evaluate the EM resistance of the metal wiring (hereinafter, referred to as a to-be-evaluated device) included in a semiconductor integrated circuit, etc. Various EM evaluation circuits and EM evaluation methods have conventionally been proposed. In load testing for applying a load current to the to-be-evaluated device, a DC (Direct Current), a unidirectional AC pulse current, a bidirectional AC pulse current, etc. can be used as the load current. An EM evaluation circuit for evaluating the EM resistance using a bidirectional AC pulse current is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-26535.
In this EM evaluation circuit, a plurality of transistors operate in response to a pulse voltage output by an oscillation circuit. Upon this, the bidirectional AC pulse current at the same frequency as that of the oscillation circuit flows to the to-be-evaluated device. If this pulse current flows to the to-be-evaluated device for a predetermined period of time, an EM phenomenon occurs, and the to-be-evaluated device is deteriorated, resulting in that the electric resistance value between both ends of the to-be-evaluated device varies. By obtaining the amount of the variation in the electric resistance value of the to-be-evaluated device, the EM resistance of the to-be-evaluated device can be evaluated. In order to achieve this, measurement terminals for measuring the electric resistance are connected respectively to both ends of the to-be-evaluated device.
Load currents flowing to the to-be-evaluated device are illustrated usually in the form of rectangular pulses, as shown in FIG. 3. However, if the frequency of such load currents is several hundred MHz, the waveform of the pulses is distorted, i.e. the deterioration of the waveform occurs, as shown in FIG. 4. This is because the parasitic capacity of several dozen pF at the measurement terminals connected to the to-be-evaluated device is charged with electricity or discharges. In the state where the waveform is being deteriorated, the current density of the load current gets lower. As a result of this, the value of the current density of the load current may become lower than a reference value which is set for the load testing. Furthermore, in the case where the waveform is extremely deteriorated, the value of the frequency of the load current may deviate from the value of the oscillation frequency. Accordingly, the EM evaluation circuit disclosed in the above publication has such a problem that the load testing is performed with low accuracy.
The present invention has been made in consideration of the above. It is accordingly an object of the present invention to provided an electromigration evaluation circuit which can eliminate the waveform of a load current from being deteriorated, during a test for evaluating electromigration resistance of a to-be-evaluated device.
Another object thereof is to provide an electromigration evaluation circuit which can accurately measure electromigration resistance of a to-be-evaluated device.
In order to achieve the above objects, according to the first aspect of the present invention, there is provided an electromigration evaluation circuit for use in evaluating electromigration resistance of a to-be-evaluated device, the circuit comprising:
two measurement terminals connected to both ends of the to-be-evaluated device;
a current applying circuit for applying a load current to the to-be-evaluated device; and
two switch circuits for electrically connecting or disconnecting between the to-be-evaluated device and the two measurement terminals, in response to a predetermined control signal.
According to the above structure, while applying a load current to the to-be-evaluated device, the two terminals and the to-be-evaluated device can electrically be disconnected from each other, so that the parasitic capacity of the terminals are not effected by the load current. Alternatively, the two terminals and the to-be-evaluated device can be electrically connected with each other, so as to externally measure the physical characteristics of the to-be-evaluated device. In the simple structure of the electromigration evaluation circuit, the electromigration resistance of the to-be-evaluated device can be measured without any effect of the parasitic capacity at the terminals.
Each of the switch circuits may:
operate in accordance with a level of the predetermined control signal;
electrically disconnect the to-be-evaluated device from the two measurement terminals, in a case where a first control signal at a level representing that the current applying circuit applies a load current to the to-be-evaluated device is sent; and
electrically connect the to-be-evaluated device with the measurement terminals, in a case where a second control signal at a level representing that electric characteristics of the to-be-evaluated device are measured is sent.
Each of the switch circuits may include
a transfer gate having a current path, connected between the to-be-evaluated device and a corresponding one of the measurement terminals, and a control terminal to which the first or second control signal is applied.
The current applying circuit may include:
a pulse generator which generates a pulse signal;
a waveform adjustment circuit which adjusts a waveform of the generated pulse signal; and
four transistor circuits each of which is connected to either end of the to-be-evaluated device, receives output signals respectively from the pulse generator and waveform adjustment circuit, and applies an alternating current to the to-be-evaluated device.
In order to achieve the above objects, according to the second aspect of the present invention, there is provided an electromigration evaluation circuit comprising:
a current applying circuit for applying a current between both ends of a to-be-evaluated device;
a first path one end of which is connected to one end of the to-be-evaluated device, and other end of which is connected to a first measurement terminal;
a second path one end of which is connected to other end of the to-be-evaluated device, and other end of which is connected to a second measurement terminal;
two switch circuits which intervene between the respective first and second paths; and
a controller which controls the two switch circuits to
electrically connect the first and second measurement terminals to the to-be-evaluated device, in a case where electric characteristics of the to-be-evaluated device are measured, and
electrically disconnect the first and second measurement terminals from the to-be-evaluated device, in a case where the current applying circuit applies a current to the to-be-evaluated device.
The controller may generate a control signal for controlling operations of the switch circuits, and send the generated control signal thereto; and
each of the switch circuits may electrically connect or disconnect each of the first and second measurement terminals to/from the to-be-evaluated device, in accordance with the sent control signal.
Each of the switch circuits may include
a transfer gate having a current path, connected between the to-be-evaluated device and a corresponding one of the measurement terminals, and a control terminal to which the control signal is applied.
The current applying circuit may include:
a pulse generator for generating a pulse signal;
a waveform adjustment circuit for adjusting a waveform of the generated pulse signal; and
four transistor circuits each of which is connected of either end of the to-be-evaluated device, receives output signals respectively from the pulse generator and waveform adjustment circuit, and applies an alternating current to the to-be-evaluated device.
In order to achieve the above objects, according to the third aspect of the present invention, there is provided a circuit comprising:
first and second measurement terminals;
first and second nodes between which a to-be-evaluated device is connected;
an AC circuit which is connected to the first and second nodes and flows current through said to-be-evaluated device; and
a switch circuit which electrically connecting or disconnecting between said first measurement terminal and said first node and between said second measurement terminal and said second node.
The switch circuits may operate in accordance with a predetermined control signal;
electrically disconnects said to-be-evaluated device from said two measurement terminals, in a case where the predetermined control signal representing that said current applying circuit applies a load current to said to-be-evaluated device is sent; and electrically connects said to-be-evaluated device with said measurement terminals, in a case where the predetermined control signal representing that electric characteristics of said to-be-evaluated device are measured is sent.
The switch circuit may include: a transfer gate having a current path, connected between said to-be-evaluated device and a corresponding one of said measurement terminals, and a control terminal to which the first or second control signal is applied.
The current supplying circuit may include: a pulse generator which generates a pulse signal; a waveform adjustment circuit which adjusts a waveform of the generated pulse signal; and four transistor circuits each of which is connected to either end of said to-be-evaluated device, receives output signals respectively from said pulse generator and waveform adjustment circuit, and applies an alternating current to said to-be-evaluated device.
In order to achieve the above objects, according to the fourth aspect of the present invention, there is provided an electromigration determining method comprising:
providing a body having an element whose electromigration is to be determined, two externally accessable terminals, an externally controllable switch circuit provided between said element and two terminals, and an externally controllable current supplying circuit;
controlling the switch circuit to connect both ends of the element to the terminals;
measuring electrical characteristics of the element through the terminals to obtain a first measurement value;
controlling the switch circuit to disconnect both ends of the element from the terminals;
controlling the current supply circuit to supply current flow between both ends of the element;
controlling the switch circuit to connect both ends of the element to the terminals;
measuring electrical characteristics of the element through the terminals to obtain a second measurement value; and
determining the electromigration of the element through the first and second measurement values.
The method may further comprises controlling the switch circuit and the current supplying circuit by supplying a control signal.
The controlling the current supply circuit comprises to supply rectangular AC wave current.